The present invention relates to mobile terminals and methods for determining mobile terminal location and, more particularly, to apparatus and methods for determining mobile terminal location based on Global Positioning System (GPS) signals.
Many mobile terminals, such as cellular mobile terminals, personal digital assistants (PDAs), laptop computers, and the like, are now equipped with GPS receivers. GPS is a space-based radio triangulation system using a constellation of satellites in orbit around the earth. A GPS receiver triangulates its position based on timing of radio signals it receives from various ones of the satellites and the known location of those satellites.
Determining the position of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process can take several minutes, as the duration of the GPS positioning process is generally dependent upon how much information a GPS receiver has initially. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have knowledge of its own approximate location, then it may not require significantly more time to acquire and lock onto GPS signals from the visible satellites. The process of monitoring GPS signals can be significantly affected by environmental factors. For example, GPS signals that may be easily acquired in the open typically become harder or impossible to acquire when a receiver is within a building, a vehicle, and/or under foliage.
In order to improve GPS receiver performance, techniques have been developed to provide GPS receivers with assistance information, e.g., time and position estimates, satellite ephemeris and clock information, and visible satellite list (which generally varies with the location of the mobile terminal), which can enable a GPS receiver to expedite its acquisition of GPS signals and associated position determination. Such assistance information may be transmitted, for example, from a terrestrial cellular communication system. Assistance information may not always be available, however, and a user may experience a significant increase in response time of the GPS receiver when such assistance information is not available, for example, when the user is traveling in a region not serviced by the user's cellular provider.